Polymer membrane, method for the production and use thereof

ABSTRACT

The present invention relates to an acid-doped polymer membrane based on polyazoles, a process for producing it and its use.  
     The acid-doped polymer membrane of the invention can be used in a variety of applications because of its excellent mechanical properties and is particularly useful as polymer electrolyte membrane (PEM) in PEM fuel cells.

[0001] The present invention relates to an acid-doped polymer membranebased on polyazoles, a process for producing it and its use.

[0002] The acid-doped polymer membrane of the invention can be used in avariety of applications because of its excellent chemical, thermal andmechanical properties and is particularly useful as polymer electrolytemembrane (PEM) in PEM fuel cells.

[0003] Acid-doped polyazole membranes for use in PEM fuel cells arealready known. The basic polyazole membranes are doped with concentratedphosphoric acid or sulfuric acid and act as proton conductors andseparators in polymer electrolyte membrane fuel cells (PEM fuel cells).

[0004] Due to the excellent properties of the polyazole polymer, suchpolymer electrolyte membranes can, when processed to produce amembrane-electrode unit (MEE), be used in fuel cells at continuousoperating temperatures above 100° C., in particular above 120° C. Thishigh continuous operating temperature allows the activity of thecatalysts based on noble metals present in the membrane-electrode unit(MEE) to be increased. Particularly when using reformates fromhydrocarbons, significant amounts of carbon monoxide are present in thereformer gas and these usually have to be removed by costly gastreatment or gas purification procedures. The opportunity of increasingthe operating temperature enables significantly higher concentrations ofCO impurities to be tolerated over the long term.

[0005] The use of polymer electrolyte membranes based on polyazolepolymers enables, firstly, part of the costly gas treatment or gaspurification procedures to be omitted and, secondly, the catalystloading in the membrande electrode unit to be reduced. Both areindispensible prerequisites for large-scale use of PEM fuel cells, sinceotherwise the costs of a PEM fuel cell system are too high.

[0006] The acid-doped, polyazole-based polymer membranes known hithertodisplay a favorable property profile. However, owing to the applicationssought for PEM fuel cells, in particular in automobile and stationaryapplications, they still require overall improvement. Furthermore, thepolymer membranes known hitherto have a high content ofdimethylacetamide (DMAc) which cannot be removed completely by knowndrying methods.

[0007] Thus, the polyazole-based polymer membranes known hitherto stilldisplay mechanical properties which are unsatisfactory for the aboveapplication after they have been doped with acid. This mechanicalinstability is reflected in a low modulus of elasticity, a low ultimatetensile strength and a low fracture toughness.

[0008] It is an object of the present invention to provide acid-dopedpolymembranes based on polyazoles which, firstly, have improvedmechanical properties and, secondly, have the advantages of the polymermembrane based on polyazoles and allow an operating temperature above100° C. without additional humidification of the combustion gas.

[0009] We have now found that a specific after-treatment of thepolyazole-based film to be doped with acid surprisingly leads to dopedpolymer membranes having improved mechanical properties, with the goodproton conductivity being retained or even improved. In addition, theafter-treatment rids the membrane of residual organic constituents suchas dimethylacetamide (DMAc) which would otherwise reduce the catalystactivity.

[0010] The present invention provides a doped polymer membrane based onpolyazoles, obtainable by a process comprising the steps

[0011] A) casting a film using a solution of polymers based onpolyazoles in a polar, aprotic organic solvent,

[0012] B) drying the film formed in step A) until it is self-supporting,

[0013] C) treating the film obtained in step B) with a treatment liquidat a temperature in the range from room temperature to the boiling pointof the treatment liquid,

[0014] D) drying and/or dabbing the film treated according to step C) toremove the treatment liquid from step C),

[0015] E) doping the film treated according to step D) with a dopingagent.

[0016] The preparation of polymer solutions based on polyazoles has beencomprehensively described in the prior art. Thus, EP-A-0816415 describesa method of dissolving polymers based on polyazoles usingN,N-dimethylacetamide as polar, aprotic solvent at temperatures above260° C. A substantially more gentle process for preparing solutionsbased on polyazoles is disclosed in the German patent application10052237.8.

[0017] As polymers based on polyazoles, preference is given to polymerscomprising recurring azole units of the formula (I) and/or (II)

[0018] where

[0019] Ar are identical or different and are each a tetravalent aromaticor heteroaromatic group which may have one or more rings,

[0020] Ar¹ are identical or different and are each a divalent aromaticor heteroaromatic group which may have one or more rings,

[0021] Ar² are identical or different and are each a trivalent aromaticor heteroaromatic group which may have one or more rings,

[0022] X are identical or different and are each oxygen, sulfur or anamino group which bears a hydrogen atom and a group having 1-20 carbonatoms, preferably a branched or unbranched alkyl or alkoxy group, or anaryl group as other radical.

[0023] Preferred aromatic or heteroaromatic groups are derived frombenzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane,diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,pyridine, bipyridine, anthracene and phenanthrene, all of which may alsobe substituted.

[0024] Ar¹ can have any substitution pattern; in the case of phenylene,for example, Ar¹ can be ortho-, meta-or para-phenylene. Particularlypreferred groups are derived from benzene and biphenylene, each of whichmay also be substituted.

[0025] Preferred alkyl groups are short-chain alkyl groups having from 1to 4 carbon atoms, e.g. methyl, ethyl, n-propyl or isopropyl andtert-butyl groups.

[0026] Preferred aromatic groups are phenyl or naphthyl groups. Thealkyl groups and the aromatic groups may be substituted.

[0027] Preferred substituents are halogen atoms such as fluorine, aminogroups or short-chain alkyl groups such as methyl or ethyl groups.

[0028] If polyazoles comprising recurring units of the formula (I) areused for the purposes of the present invention, the radicals X within arecurring unit should be identical.

[0029] The polyazoles used according to the invention can in principlealso have different recurring units which differ, for example, in theirradical X. However, they preferably have only identical radicals X in arecurring unit.

[0030] In a preferred embodiment of the present invention, the polymercomprising recurring azole units is a copolymer comprising at least twounits of the formula (I) and/or (II) which differ from one another.

[0031] In a particularly preferred embodiment of the present invention,the polymer comprising recurring azole units is a polyazole comprisingonly units of the formula (I) and/or (II).

[0032] The number of recurring azole units in the polymer is preferablygreater than or equal to 10. Particularly preferred polymers comprise atleast one 100 recurring azole units. For the purposes of the presentinvention, polymers comprising recurring benzimidazole units arepreferably used. An example of an extremely advantageous polymercomprising recurring benzimidazole units is represented by the formula(III):

[0033] where n is an integer greater than or equal to 10, preferablygreater than or equal to 100.

[0034] Casting of a polymer film from a polymer solution according tostep A) is carried out by means of measures known per se from the priorart.

[0035] Drying of the film in step B) is carried out at temperatures inthe range from room temperature to 300° C. Drying is carried out underatmospheric pressure or reduced pressure. The drying time depends on thethickness of the film and is preferably from 10 seconds to 24 hours. Thefilm which has been dried in step B) is subsequently self-supporting andcan be processed further. Drying is carried out by means of dryingmethods customary in the films industry.

[0036] As a result of the drying procedure carried out in step B), thepolar, aprotic organic solvent is very largely removed. Thus, theresidual content of the polar, aprotic organic solvent is usually 10 -23%.

[0037] A further lowering of the residual solvent content to below 2% byweight can be achieved by increasing the drying temperature and dryingtime, but this significantly prolongs the subsequent doping of the film,for example with phosphoric acid. A residual solvent content of 5-15% isthus useful for reducing the doping time.

[0038] The treatment of the film which has been dried in step B) uses atreatment liquid and is out in the temperature range from roomtemperature (20° C.) and the boiling point of the treatment liquid atatmospheric pressure.

[0039] As treatment liquid for the purposes of the invention and for thepurposes of step C.), use is made of solvents which are liquid at roomtemperature [i.e. 20° C.] selected from the group consisting ofalcohols, ketones, alkanes (aliphatic and cycloaliphatic), ethers(aliphatic and cycloaliphatic), esters, carboxylic acids, with theabovementioned group members being able to be halogenated, water,inorganic acids (e.g. H₃PO₄, H₂SO₄) and mixtures thereof.

[0040] Preference is given to using C1-C10 alcohols, C2-C5 ketones,C1-C10-alkanes (aliphatic and cycloaliphatic), C2-C6-ethers (aliphaticand cycloaliphatic), C2-C5 esters, C1-C3 carboxylic acids,dichloromethane, water, anorganic acids (e.g. H₃PO₄, H₂SO₄) and mixturesthereof.

[0041] The treatment liquid introduced in step C) can be removed bymeans of the drying procedure carried out in step D). The dryingprocedure depends on the partial vapor pressure of the treatment liquidchosen. Drying is usually carried out at atmospheric pressure andtemperatures in the range from 20° C. to 200° C. More gentle drying canalso be carried out under reduced pressure. In place of drying, themembrane can also be dabbed to free it of excess treatment liquid instep D). The order is not critical.

[0042] In step E), the doping of the film obtained from step C) or D) iscarried out. For this purpose, the film is wetted with a doping agent orlaid in this. As doping agent for the polymer membrane of the invention,use is made of acids, preferably all known Lewis and BrØnsted acids, inparticular inorganic Lewis and BrØnsted acids.

[0043] Apart from these abovementioned acids, the use of polyacids, inparticular isopolyacids and heteropolyacids, and of mixtures of variousacids is also possible. For the purposes of the present invention,heteropolyacids are inorganic polyacids which have at least twodifferent central atoms and are in each case partial mixed anhydridesformed from weak, polybasic oxo acids of a metal (preferably Cr, Mo, V,W) and a nonmetal (preferably As, I, P, Se, Si, Te). They include, interalia, 12-molybdophosphoric acid and 12-tungstophosphoric acid.

[0044] Doping agents which are particularly preferred for the purposesof the invention are sulfuric acid and phosphoric acid. A veryparticularly preferred doping agent is phosphoric acid (H₃PO₄).

[0045] The polymer membranes of the invention are doped. For thepurposes of the present invention, doped polymer membranes are polymermembranes which, owing to the presence of doping agents, displayincreased proton conductivity compared to the undoped polymer membranes.

[0046] Processes for preparing doped polymer membranes are known. In apreferred embodiment of the present invention, they are obtained bywetting a film of the polymer concerned with concentrated acid, forexample with highly concentrated phosphoric acid, for a suitable time,preferably 5 minutes -96 hours, particularly preferably 1 -72 hours, attemperatures in the range from room temperature to 100° C. andatmospheric or superatmospheric pressure.

[0047] The conductivity of the polar membrane of the invention can beinfluenced via the degree of doping. The conductivity increases withincreasing concentration of doping agent until a maximum value has beenreached. According to the invention, the degree of doping is reported asmol of acid per mol of recurring units of the polymer. For the purposesof the present invention, a degree of doping of from 3 to 15, inparticular from 6 to 12, is preferred.

[0048] The polymer membrane of the invention has improved materialsproperties compared to the previously known doped polymer membranes. Inparticular, they have very good mechanical properties and perform betterthan untreated membranes.

[0049] The polymer membranes of the invention display improved protonconductivity compared to untreated membranes.

[0050] Possible applications of the doped polymer membranes of theinvention include, inter alia, use in fuel cells, in electrolysis, incapacitors and in battery systems. Owing to their property profile, thedoped polymer membranes are preferably used in fuel cells.

[0051] The present invention also relates to a membrane-electrode unitcomprising at least one polymer membrane according to the invention. Forfurther information on membrane-electrode units, reference may be madeto the specialist literature, in particular the patents U.S. Pat. No.4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4333,805. Thedisclosure in the abovementioned references [U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805] in respect of thestructure and production of membrane-electrode units is incorporated byreference into the present description.

[0052] The invention is illustrated below by means of examples and acomparative example, without the invention being restricted to theseexamples.

EXAMPLES

[0053] Untreated film:

[0054] The untreated films were laid in 85% strength H₃PO₄ for 96 hours.Prior to doping with H₃PO₄, the H₂O content and residual solvent contentof the film are determined by Karl Fischer (KF) titration. The watercontent of the film is determined directly as follows by KF titrationusing a Mettler-Toledo apparatus. The sample, which is present in aclosed sample vial, is heated to 250° C. and dried at this temperature.The gas liberated in this way is passed directly into a closed titrationvessel and analyzed by means of a Karl Fischer [KF] reagent. Apart fromthe determination of the water content, the residual solvent content isdetermined by determining the weight before and after drying.

[0055] Washing with H₂O and subsequent thermal drying:

[0056] The films were boiled in water for 1 hour. The water bath wasthen changed and the films were boiled for a further hour. The filmswere subsequently rinsed with fresh water and finally dried at 160° C.for 3 hours. H₂0 content and residual solvent content were determined onthe treated films by KF titration. The membranes were obtained by dopingthe films in 85% strength H₃PO₄ for 96 hours.

[0057] Washing with H₂O:

[0058] The films were boiled in water for 1 hour. The water bath is thenchanged and the films are boiled for a further hour. The films weresubsequently dabbed with a cloth and used further in moist form. H₂Ocontent and residual solvent content of the film were determined by KFtitration. The membranes were doped in 85% strength H₃PO₄ for 96 hours.

[0059] Washing with methanol:

[0060] The films were placed in methanol and boiled under reflux for 2hours (beginning when the methanol started to boil). The films weretaken out and dried firstly for 1 minute in air minute in air and thenat 100C. under reduced pressure in a drying oven for 2 hours. H₂Ocontent and residual organic solvent content of the film were determinedby KF titration. The membranes were doped in 85% strength H₃PO₄ for 96hours.

[0061] Washing with acetone:

[0062] The films were placed in acetone and boiled under reflux for 2hours (beginning when the acetone started to boil). The films were thendried firstly for 1 minute in air at RT and subsequently at 10020 C.under reduced pressure in a drying oven for 2 hours. H₂O content andresidual solvent content of the film were determined by KF titration.The membranes were doped in 85% strength H₃PO₄ for 96 hours.

[0063] FIG. 1 shows the result of the KF titration. The residual organicsolvent is removed completely by washing with water. The residualorganic solvent content is reduced from 16.6% to 3.7 or 2.3% by washingwith acetone or with methanol, respectively.

[0064] FIG. 2 shows a proton conductivity which is improved by 10% evenat room temperature and is retained or improved further at elevatedtemperature. The specific conductivity is measured by means of impedancespectroscopy in a 4-pole arrangement in the potentiostatic mode usingplatinum electrodes (wire, 0.25 mm diameter). The distance between thecurrent collector electrodes is 2 cm. The spectrum obtained is evaluatedusing a simple model consisting of a parallel arrangement of an ohmicresistance and a capacitor. The specimen cross section of the membranedoped with phosphoric acid is measured immediately before mounting ofthe specimen. To measure the temperature dependence, the measuring cellis brought to the desired temperature in an oven and the temperature isregulated via a Pt-100 temperature sensor positioned in the immediateproximity of the specimen. After reaching the temperature, the specimenis maintained at this temperature for 10 minutes before commencement ofthe measurement.

[0065] To determine the mechanical properties, uniaxial tensile testsare carried out on tension bars. A Zwick tester equipped with a 100 Nload cell and a heatable oven is used for this purpose. The length ofspecimen between the chucks is 10 cm and the separation velocity is setat 50 mm/min. The deformation is determined directly via the distance oftravel. The tensile tests on membranes doped with phosphoric acid arecarried out at 100° C. To calculate the stress automatically, the crosssection of each specimen is determined and entered before commencementof the test. To determine mean values of modulus of elasticity, tensilestrength, elongation at break and rupture energy (toughness), at least 5measurements are carried out on each membrane.

[0066] The results of the tensile tests on the polymer membranesaccording to the invention compared to untreated membranes are shown byway of example in FIG. 3. It can be seen from the figure that a membranewashed with water has the highest elongaton at break and the highesttensile stress at break.

[0067] An untreated membrane displays an elongation at break of 55%while a membrane according to the invention has an elongation at breakin the range from 58% to 75%.

[0068] The results of the tensile tests are summarized in Table 1. TABLE1 Results of the tensile tests on membranes after different washingprocedures compared to an untreated membrane. Error Error in Error inTensile in tensile Elongation at elongation Rupture Error in E Estrength strength break at break energy rupture energy Washing Method[MPa] [MPa] [MPa] [MPa] [%] [%] [kJ/m²] [kJ/m²] untreated 4.7 0.7 1.50.13 55 5 54 5 washed with water 5 0.55 1.7 0.25 71 11 74.5 18 washedwith 5.45 0.4 1.55 0.14 64.7 6 63 8.8 acetone washed with 5.3 0.5 1.360.22 61.2 13 54 18.6 methanol

1. A doped polymer membrane based on polyazoles, obtainable by a processcomprising the steps A) casting a film using a solution of polymersbased on polyazoles in a polar, aprotic organic solvent, B) drying thefilm formed in step A) until it is self-supporting, C) treating the filmobtained in step B) with a treatment liquid at a temperature in therange from room temperature to the boiling point of the treatmentliquid, D) drying and/or dabbing the film treated according to step C)to remove the treatment liquid from step C), E) doping the film treatedaccording to step D) with a doping agdent.
 2. A polymer membrane asclaimed in claim 1 which is proton-conducting.
 3. A polymer membrane asclaimed in claim 1, wherein the polymer based on polyazoles comprisesrecurring azole units of the formula (I) and/or (II)

where Ar are identical or different and are each a tetravalent aromaticor heteroaromatic group which may have one or more rings, Ar¹ areidentical or different and are each a divalent aromatic orheteroaromatic group which may have one or more rings, Ar¹ are identicalor different and are each a trivalent aromatic or heteroaromatic groupwhich may have one or more rings, X are identical or different and areeach oxygen, sulfur or an amino group which bears a hydrogen atom and agroup having 1-20 carbon atoms, preferably a branched or unbranchedalkyl or alkoxy group, or an aryl group as other radical.
 4. A polymermembrane as claimed in claim 1, wherein the polymer comprising recurringazole units is a copolymer comprising at least two units of the formula(I) and/or (II) which differ from one another.
 5. A polymer membrane asclaimed in claim 1, wherein the polyazole consists only of units of theformula (I) and/or (II).
 6. A polymer membrane as claimed in claim 1,wherein the polyazole is a polymer comprising recurring benzimidazoleunits of the formula (III)

where n is an integer greater than or equal to 10, preferably greaterthan or equal to
 100. 7. A polymer membrane as claimed in claim 1,wherein the degree of doping is from 3 to 15 mol of acid per mol ofrecurring unit of the polymer.
 8. A membrane-electrode unit comprisingat least one polymer membrane as claimed in claim 1 and at least oneelectrode.
 9. A polymer electrolyte fuel cell comprising at least onemembrane-electrode unit as claimed in claim 8.